1. Field of the Invention
The present invention relates to magnetic resonance imaging for imaging the inside of a subject on the basis of a magnetic resonance phenomenon occurring in the subject. More particularly, this invention is concerned with a magnetic resonance imaging (MRI) system and magnetic resonance (MR) imaging method suitable for imaging of a tissue or blood flow in a subject composed of nuclear spins whose transverse (spin-spin) relaxation time (T.sub.2) is rather short.
2. Description of the Related Art
Magnetic resonance imaging is a technique for magnetically exciting nuclear spins in a subject positioned in a static magnetic field by applying a radio-frequency signal with the Larmor frequency, and reconstructing an image using an MR signal induced with the excitation or providing a spectrum of an MR signal.
For imaging the vessels in the lungs or the vessels in the liver (portal vein) according to magnetic resonance imaging, various requirements must be satisfied. One of the requirements is to improve a signal-to-noise ratio by raising signal levels representing a vascular image. Another one is to minimize artifacts caused by body motions.
As a technique coping with the former or a technique for raising a signal, there is a technique of averaging n (larger than 1) MR data items pixel by pixel. For carrying out averaging, the number of shots or the number of scans is increased in order to increase the number of data items per pixel. Thus, the frequency of accumulation per pixel is increased. A phase-encoding direction, that is, a direction in which the distribution of spins is phase-encoded in order to acquire an MR signal used for averaging is set to a certain direction.
Moreover, for satisfying the latter requirement, that is, for suppressing occurrence of body-motion artifacts, there is an approach in which a patient is asked to hold his or her breath. This makes it possible to minimize body-motion artifacts caused by the motion of the lungs.
However, when a person's breath is held a plurality of times and MR data items acquired during periods of breath hold are used to produce an image, the image may be blurred due to the influence of body-motion artifacts caused by the movement of the patient's body. For this reason, a patient is usually asked to hold his or her breath only once. Efforts are made to increase the frequency of accumulation required for averaging during the one breath hold.
However, especially when fast spin echo imaging (FSE) or echo planar imaging (EPI) is adopted, even if the approach to one breath hold and the averaging technique are adopted, the running state of components whose time T.sub.2 is rather short (or ranges from 100 to 200 msec) (a blood flow, or especially, a vessel in the lungs or a vessel in the liver (portal vein) or a vas) cannot be visualized successfully. This combination is therefore unsatisfactory in terms of visualization ability.
This is attributable to the fact that the half-width of a function of an MR signal induced by components whose time T.sub.2 is rather short (hereinafter, simply, a blood flow) in relation to pixel locations in a phase-encoding direction is large (stretched), and a whole image is blurred in the phase-encoding direction.
When an image is blurred in the phase-encoding direction, pixel values expressing an image of a blood flow crossing (orthogonal to) the phase-encoding direction and those expressing surrounding tissues are added up and averaged. This results in deteriorated resolution. In other words, in an image, it becomes hard to discriminate a blood flow running in a direction crossing the phase-encoding direction from surrounding tissues.
When MR data acquired with a phase-encoding direction set to a given direction is subjected to averaging, the resolution of a blood flow running in the phase-encoding direction improves. However, the resolution of a blood flow running in any other direction remains low because the values of blurred pixels are merely averaged.
In the case of known averaging, it is hard to produce an image depicting vertically-and-laterally-running blood flows with the running directions of the blood flows clearly discernible without the loss of running information. Blood flows running in directions other than a phase-encoding direction are likely to be missing out of an image, and can sometimes not be identified by looking carefully at the image. This problem is serious in imaging of a blood flow composed of spins whose time T.sub.2 is rather short.